python310Packages.python-lsp-server: Add meta.mainProgram (#221728)

The `python-lsp-server` package has its main binary as `pylsp`. The
`mainProgram` meta attribute should reflect such to enable use of
`lib.getExe`.

pkgs/development/python-modules/python-lsp-server/default.nix

@@ -161,5 +161,6 @@ buildPythonPackage rec {
     changelog = "https://github.com/python-lsp/python-lsp-server/blob/v${version}/CHANGELOG.md";
     license = licenses.mit;
     maintainers = with maintainers; [ fab ];
+    mainProgram = "pylsp";
   };
 }